This invention relates to an improved carton and method for manufacture thereof, and more particularly relates to the manufacture of cartons from card and board stock at least which is coated on at least one surface with a Polyethylene terephthalate (PET) layer.
It is to be mentioned that currently single sided PET clay card stock is used in the manufacture of rectangular and square tray-like cartons for containment of foodstuffs which are to be heated prior to consumption either in a conventional oven or in microwave oven, and therefore such cartons are commonly known as double-ovenable cartons.
It has been established that PET has desirable heat resistance properties in that its melting point is approximately 245.degree. C. and therefore can easily withstand the temperatures to which foodstuffs may be raised in such ovens without decomposing or melting and thus impairing said foodstuffs.
In an alternate but associated technical field, cups and other cartons with a cylindrical, or partially or completely arcuate base shape, are conventionally manufactured using board and card stock with at least one coating of polyethylene (PE) thereon and are typically of two-piece construction, in particular a base portion and a side wall portion. PE has excellent fluid retention characteristics, and is therefore ideal for the manufacture of cups and the like which commonly contain cold beverages, but its low melting point (106-109.degree. C.) precludes its use in double-ovenable carton manufacture.
Until now, it has been considered impossible to manufacture a two-piece carton with a partially or completely arcuate base shape from card stock with at least one PET coating on a particular surface of said card stock, and this consideration has been primarily based on the assumption that the various process steps which are conducted on PE coated card during the manufacture of cups and the like could not be applied to PET coated card without significant material degradation.
The perceived difficulties associated with cup technology as applied to PET carton manufacture are fundamentally related to the bonding of the base portion to the side wall portion to provide a substantially hermetically sealed base construction which adequately retains fluid within the carton. This is essential when it is considered that a particular application of the carton according to the invention is to contain either frozen or chilled food products which may have a high water content.
In particular, it is known that the PET coating on conventional card stock has a tendency to shrink back on itself and thus revealing the card substrate to which said coating was applied. This is completely unacceptable for the reasons mentioned above and in that the fluid repellency of the card stock provided by the PET layer is unacceptably compromised.
Although not an essential requirement of the manufacture of PE cups and the like, it is preferable that the said cups be manufactured without the need for separate adhesives and thus heat is generally applied, either by hot air or by sonication to the card to melt the PE layer which ultimately bonds the base portion of the cup to the side walls, and also bonds the seam where the side wall portion is overlapped. This method of bonding is most expedient, increases the efficiency of the process and the rate at which cups can be produced, and furthermore is beneficial to the fluid retention characteristics of the resulting cup because PE has a tendency to "flow" when molten as opposed to the shrinking characteristic associated with PET. The fluidity of molten PE is of advantage in two respects. Firstly, the molten PE material can flow into channels formed between the base portion and the side wall portion of the cup as it is being formed and so improve its fluid retention characteristics, and secondly, the fluid repellency of both the base portion and the side wall portion internally of the cup are not compromised significantly if the heat is inaccurately applied around the contact location between the base portion and the side wall portion.
With particular regard to the second of these advantages, it is to be pointed out that the shrinkage of PET gives rise to "pin-holing" in the PET coating, and areas of what is termed "heat splash" are visible internally of the cup on the side walls proximate the base.
Arising from the sealing difficulties mentioned above, experimentation has shown that conventional base construction as currently practised in cup manufacture, but using PET coated card can result in fluid leakage through the channels formed between the base portion and side wall portion internally of the cup. Again for the reasons mentioned above, this is unacceptable for PET cartons which are to hold fluids. Furthermore, the increased melting temperature of PET as compared to PE precludes the use of sonication as a means for melting PET.
The object of the invention is to provide a method and means for manufacture of double-ovenable two-piece cartons from card or board stock provided with a coating of PET on at least side thereof which mitigates against the disadvantages described above.
It is a further object of this invention to provide a carton manufactured from card or board stock provided with a coating of PET on at least side thereof which is of rigid and watertight construction which can nevertheless be manufactured in quick repeated succession.
It is a yet further object of this invention to provide a modified base construction for a PET two-piece construction carton which provides increased nigidity for the carton and furthermore is less prone to leakage.